Three step process for treating plastics with alkaline permanganate solutions

ABSTRACT

Process for the planting of through-holes in an epoxy printed circuit board base material in the manufacture of printed circuit boards. The treatment process is designed to increase adhesion between the epoxy board material and a subsequently deposited metal plate. The process is characterized, in part, by the use of an activated permanganate solution.

This is a divisional of co-pending application Ser. No. 173,068 filed onMar. 25, 1988, now U.S. Pat. No. 4,820,548, which is a divisional ofco-pending application Ser. No. 860,100, filed May 6, 1986, nowabandoned which is a divisional of two prior applications, includingSer. No. 614,912, filed May 29, 1984, now abandoned, and Ser. No.791,804 filed, Oct. 28, 1985, now U.S. Pat. No. 4,629,636 (which is, inturn, a divisional of Ser. No. 618,281, filed June 7, 1984, now U.S.Pat. No. 4,592,852).

BACKGROUND OF THE INVENTION

The invention relates to the metal plating of plastics and, inparticular, to enhancing the adhesion of metal plating to the plasticsubstrate of electronic circuit boards by treating the board in a threestep process with an alkaline permanganate solution prior to electrolessmetal plating of the board.

The metal plating of plastic parts is well-known to be of considerablecommercial importance because the desirable characteristics of both theplastic and the metal are combined to offer the technical and aestheticadvantages of each. Thus, a part plated with a bright, metallic finishtakes advantage of the economies in cost and weight afforded bysubstituting molded plastic parts for metal and, additionally, theplated finishes are not as susceptible to pitting and corrosion becausethere is no galvanic reaction between a plastic substrate and a platedmetal.

An important process is the preparation of electronic circuit boardswhich requires the electroless plating of a conductive metal layer,usually copper, onto the plastic substrate of the board, such as epoxy,and for convenience the following description will relate specificallyto this process.

These boards vary in design and may have a copper layer on each surfaceof the epoxy (two-sided boards) or they can be multi layer boards whichhave a plurality of inter-leaved parallel planar copper and epoxylayers. In both type boards through-holes are drilled in the board andmetal plated to facilitate connection between the circuits on the copperlayers. The through-holes present an additional plating problem becauseresin smear on the exposed copper caused by the drilling operation actsas an insulator between the metal of the through-hole and copper layerand must be removed prior to plating. The smear is usually removed usingacid and this process degrades the physical integrity of the hole makingit difficult to metallize and provides little or no adhesive support forthe metallic deposit.

The problems in plating either the through-holes or other plastic partsof the board are well-known in the art and a number of methods have beendeveloped to improve the adhesion of the metal plating to the epoxy.These methods generally employ oxidants to etch the surface of theplastic prior to plating and include chromic acid, sulfuric acid andacidic and alkaline permanganate solutions. The toxicity of the chromiumcompounds and their potential hazards as water pollutants and the safetyprecautions needed with sulfuric acid have increased the commercial useof permanganate solutions, particularly alkaline permanganate solutions,and a number of patents have been granted in this area.

U.S. Pat. No. 3,652,351 shows the etching ofacrylonitrile-butadiene-styrene interpolymers (ABS plastics) using acomposition containing manganate and hydroxyl ions. U.S. Pat. Nos.4,042,729 and 4,054,693 disclose stable, highly active etching solutionscontaining particular ratios of manganate ions and permanganate ions bycontrolling the pH in the range of 11 to 13. U.S. Pat. No. 4,425,380 isspecifically directed to cleaning through-holes of residual manganeseprior to plating by contacting the etched plastic with a water solublecompound oxidizable by permanganate to reduce the manganese residues toa low oxidation state, e.g. SnCl2-HCl, formaldehyde, followed bycontacting with hot alkaline hydroxide. The disclosures of the abovepatents are hereby incorporated by reference.

The present invention relates to a three step process for treating theplastic surface before plating. The first step includes treating theplastic surface before plating and particularly before etching tofurther enhance the adhesive effect of the etching procedure. Thisprocedure is generally known as a solvent-etch technique and employssolvents which swell the plastic. U.S. Pat. No. 3,758,332 discloses theuse of chemicals such as methyl ethyl ketone, tetrahydrofuran, dioxane,pyridine, dimethylformamide, and an alcohol mixture comprising methylethyl ketone, ethanol and methanol as swellants for epoxy resin. U.S.Pat. No. 4,086,128 also shows pretreatment of an epoxy resin with anorganic solvent comprising alcohols, acids, esters, ketones, nitriles,nitro compounds, and polyhydric compounds such as ethylene glycol,glycerine and 1, 2-propylene glycol prior to etching with hydrogenperoxide and sulfuric acid. U.S. Pat. No. 3,865,623 shows immersion ofepoxy resin in an organic solvent such as dimethylformamide to renderthe epoxy receptive to an acid etch. The disclosures of the abovepublications are hereby incorporated by reference. The second stepincludes etching the plastic with an alkaline permanganate solution toremove smear and to improve the adhesiveness of the metal plate to theplastic board. U.S. Pat. No. 4,042,729, supra, attempted to improve theuse of alkaline permanganate solutions to treat plastic circuit boardsand notes the many problems associated with these solutions. Thus,acidic permanganate solutions are notoriously unstable, have a shortlife and rapidly decompose to manganese dioxide. Alkaline permanganatesolutions present similar problems but, according to the invention, arehighly active, easily controllable and stable provided the molar ratioof manganate ion to permanganate ion is up to about 1.2 and the pH ofthe solution is controlled in the range of 11 to 13 by the use ofbuffers or pH adjustors. The patent also provides a means forrejuvenating the bath to remove organics and produce permanganate bydisproportionation of manganate comprising bubbling CO₂ into the bath tolower the pH from 12.5 to 11 to 11.5, heating to form permanganate ionsand manganese dioxide from the manganate ions, cooling, precipitatingcarbonates and filtering to remove the manganese dioxide and carbonates.The preparation and control of the compositions and rejuvenatingprocedures of this patent are unwieldly and time consuming and there isa need for improved alkaline permanganate etching solutions and methodsfor using the solutions to etch plastic substrates.

The third step includes neutralization with a reductant to solubilizemanganese residues by lowering their oxidation state.

SUMMARY OF THE INVENTION

It has now been discovered that the adhesiveness of metal plating and,in particular, electroless metal plating, to plastic substrates can beenhanced by a three step process.

The first step is contacting the plastic substrate for an effective timewith an alkaline composition preferably comprising:

(a) a compound represented by the general formula, ##STR1## and (b) acompound represented by the general formula,

    R.sub.3 --OCH.sub.2 CH.sub.2)n O--R.sub.4

wherein R₁, R₂, R₃, and R₄ are independently selected from the groupconsisting of hydrogen atoms, aryl

groups and alkyl groups of 1-4 carbon atoms, m is 0 to 2 and n is 2 to5; the compositions comprising, by weight about 10 grams/liter (g/l) tosaturation compound (a) and about 10 g/l to saturation compound (b).

The pH of the composition is preferably highly alkaline, e.g., greaterthan 10 and more preferably greater than 13, e.g., 14. It is preferredto employ a source of hydroxyl ions such as an alkali metal hydroxideand may be present in an amount of about 5 g/l to 200 g/l or more.Compound (a) is preferably propylene glycol monomethyl ether andcompound (b) dimethoxy tetra ethylene glycol, with compounds (a) and (b)each being present in the solution in an amount of about 40 g/l to 120g/l.

The composition is employed at an elevated temperature and is, ingeneral, about 90° F. (32° C.) to the lesser of the boiling point of thesolution and the softening point of the plastic. For epoxy boards thetemperature is preferably about 140 to 150° F. (60 to 66° C.).(Temperatures given in degrees Celsius are approximate, i.e., thenearest whole degree Celsius to the corresponding temperature in degreesFahrenheit, and are provided solely as a convenience to the reader.)

The effective contact time will vary with the concentration andtemperature of the solution and the plastic substrate being treated andin general will not exceed about 30 minutes, preferably less than 10minutes, e.g., 5 minutes. For epoxy boards which are to be etched in thesecond step with alkaline permanganate solutions a contact time of about2 to 10 minutes at a temperature of about 140-150° F. (60-66° C.) hasbeen found to provide excellent results.

The second step of the process uses an alkaline permanganate etchingcomposition comprising permanganate ions and preferably including asecondary oxidant capable of oxidizing manganate ions to permanganateions which is highly effective to treat plastic substrates prior toelectroless metal deposition of the plastic. The improved permanganatecompositions when used to treat plastics such as epoxy and other resinsexhibit excellent stability and longterm operating life. The formationof undesirable lower oxidation state manganese products such asmanganese dioxide, is lowered and additionally, the activity of thesolution is also unexpectedly enhanced and shorter treatment times aregenerally required than with prior art compositions.

It is another important unexpected feature of the invention that theimproved alkaline permanganate etching process proceeds without anysignificant adverse operating effects which would, for example, increasethe number of treated parts being rejected for failing to meetspecifications. While no theory can be advanced for the unexpectedresults achieved when using the compositions of the invention, itremains that the invention provides an etchant composition and procedurefor preparing plastic substrates for metallization superior to any knownto the prior art.

The alkaline permanganate solutions are well-known in the art andsuitable compositions are shown in U.S. Pat. Nos. 4,042,729 and4,425,380. In general, the permanganate component is present in anamount of about 1 gram per liter (g/l) to the limit of its solubility inthe medium and the pH of the composition is alkaline and preferablyabove about 10, more preferably above about 13, e.g., between 13 and 14.The amount of the preferred secondary oxidant component having anoxidation potential greater than the oxidation potential of the alkalinepermanganate solution may vary widely from about 0.5 gram equivalent ofoxidant per gram equivalent of the manganate ion concentration to thestoichiometric amount, or more.

The permanganate composition is employed at an elevated temperature andis, in general, about 90° F. (33° C.) to the lesser of the boiling pointof the solution and the softening point of the plastic. For epoxy boardsthe temperature is preferably about 140 to 160° F. (61 to 72° C.).(Temperatures given in degrees Celsius are approximate, i.e., thenearest whole degree Celsius to the corresponding temperature in degreesFahrenheit, and are provided solely as a convenience to the reader.

The effective contact time of step two (permanganate etching) will varywith the concentration and temperature of the permanganate solution andthe plastic substrate being treated and in general will not exceed about30 minutes, preferably 5 to 15 minutes, e.g., 10 minutes. For epoxyboards a contact time of about 8-12 minutes at a temperature of about145-155° F. (64-69° C.) has been found to provide excellent results.

The third step of the process includes neutralization with a reductantto solubilize manganese residues by lowering their oxidation state.

In general and for purposes of illustration, the process of using thecompositions of the invention in manufacturing printed circuit boardsand, in particular, boards containing through-holes, is a sequence ofsteps commencing with a commercially available laminate ormulti-laminate made from, e.g., paper-epoxy or glass-epoxy material. Apredesigned series of through-holes is formed in the board in anyconventional manner, then preferably etched with an acid to remove anyepoxy smear over the exposed metal laminate surface and to etch-back theepoxy layer, and water rinsed. This is followed in step one of theprocess by contacting the board with the solvent composition of theinvention to enhance the effect of the subsequent oxidative etching instep two. Similar step one type swellant compositions which are suitableto enhance the effect of the subsequent permanganate etching step aredescribed in U.S. Pat. Nos. 3,758,332; 3,865,623 and 4,086,128, thedisclosures of which are hereby incorporated by reference. After waterrinsing, the board is etched with the alkaline permanganate solution ofstep two of the invention and water rinsed. Neutralization with areductant to solubilize manganese residues by lowering their oxidationstate is then performed as step three and the board rinsed providing aboard having all the copper layers in the through-holes exposed and theepoxy etched ready for electroless metal plating using conventionalprocedures. A preferred step is to now dissolve some of the glass fibersin the epoxy using, for example, acid fluorides, and to pretreat theboard by cleaning with a composition such as ENPLATE® PC-475 to removehydrocarbon soils and oxides and micro-etching of the copper using anetchant such as ENPLATE AD-485. Thereafter the board is immersed in acatalyst, such as a tin-palladium solution, which conditions the surfaceof the epoxy for electroless copper plating. ENPLATE Activator 444 soldby Enthone, Incorporated is exemplary of this type catalyst. Following awater rinse, the laminate is immersed in an accelerator such as ENPLATEPA-491 to activate the catalyst by freeing the metal palladium ions onthe board. After a water rinse, the board is dried and is immersed in anelectroless copper plating solution for a period of time sufficient toplate copper to the desired thickness on the surfaces and to plate thesurfaces of the holes to form through-hole connections between thelaminate surfaces. ENPLATE CU-700 and other similar plating compositionsmay be employed. The boards may then be electroplated using conventionaltechniques if a thicker coating is desired.

DETAILED DESCRIPTION OF THE INVENTION

The compositions of the invention have been found to be particularlyuseful for treating epoxy resins although other suitable polymericresins may also be conditioned. The electronic circuit boards arecommercially available and are of known structure and composition andinclude paper-epoxy and glass-epoxy laminates. In general, an epoxyresin means a polymeric compound containing at least two epoxy groups inone molecule and synthetic resins formed by ring-opening reactions ofthe epoxy groups of the compounds. An epoxy resin extensively used inthe printed circuit industry is a cured epoxy resulting from acondensation product of epichlorohydrin and bisphenol A. A board termedFR-4, which is a fire retardant epoxy resin fiber glass cloth laminate,has demonstrated to be effectively treated by the three step process ofthe invention.

The epoxy resin may contain glass fibers, paper, synthetic fibers,carbon black, alumina powders, silica powders, wax, etc. as fillers,pigments, mold release agents, reinforcing agents, etc., or can be usedtogether with phenol resins, urea resins, melamine resins, and the like.

The alkaline pH of the solvent (step one) and permanganate (step two)compositions may be provided by any suitable source and is preferably analkaline hydroxide such as lithium, sodium, potassium, cesium, andtetraalkyl ammonium. Sodium hydroxide and potassium hydroxide arepreferred for the permanganate composition and sodium hydroxide ispreferred for the solvent composition.

The amount of hydroxyl ions in the solvent solution of step one can varywidely and is preferably about 40 to 120 g/l and most preferably about60 to 100 g/l.

The compound represented by the formula ##STR2## is preferably propyleneglycol monomethyl ether, wherein R₁ is hydrogen, R₂ is methyl. Othercompounds include propylene glycol monoethyl ether and propylene glycolmonopropyl ether. A mixture of compounds wherein R₁ is hydrogen and R₂is methyl, ethyl and propyl has provided satisfactory results. Thecompound is preferably present in the solution in an amount about 40 to120 g/l, most preferably about 60 to 100 g/l.

The compound represented by the formula

    R.sub.3 --O CH.sub.2 CH.sub.2)n O--R.sub.4

is preferably dimethoxy tetra ethylene glycol, wherein R₃ and R₄ aremethyl and n is 4. Dimethoxy tri ethylene glycol (n is 3) providessatisfactory results although the tetra ethylene glycol compound ispreferred. The compound is present in the solvent solution in an amountabout 40 to 120 g/l, most preferably about 60 to 100 g/l.

It will be appreciated by those familiar with the art that thecomponents of the solvent composition may vary widely, depending upon,for example, solubility considerations, with the understanding that theimportant criteria is the combination of compounds (a) and (b) in analkaline solution.

Other additives, such as surfactants, may be used in the compositionsfor special purposes as is known in the art.

The solvent composition media is preferably aqueous and substantiallyinert with respect to the plastic being treated. Other media may also beemployed such as alcohols with water being preferred for economicreasons and for its demonstrated effectiveness. - To practice the methodof the invention the plastic substrate is contacted with the solventcomposition at an elevated temperature for a time sufficient to renderthe surface receptive to the permanganate etching process of step two.Contacting procedures may vary widely and satisfactory results for epoxyresin are provided by immersing the part in the solvent of step one forbetween approximately 2 to 10 minutes at 140 to 150° F. (60 to 66° C.).The time and temperature will vary depending on the substrate beingtreated and the composition of the solution as will be appreciated bythose skilled in the art. Other means such as spraying, may be used fortreating the plastic part.

The treated plastic part is then ready for the second step of oxidativeetching with the hot alkaline permanganate solution. The amount ofhydroxyl ions in the permanganate solution can vary widely and ispreferably about 10 to 150 g/l, or higher, and more preferably about 20to 90 g/l, most preferably 30-70 g/l. It is preferred to measure thehydroxide ion concentration at predetermined intervals and to add morealkali as needed to maintain the desired concentration.

Any source of permanganate ion may be employed which is stable andsoluble in the solution. It is preferred to employ alkali metal oralkaline earth metal salts such as sodium, potassium, lithium, cesium,calcium, etc., with potassium permanganate being preferred because ofits availability and demonstrated effectiveness.

The amount of permanganate salt used in the solution may vary widelyfrom about 1 g/l up to the limit of solubility in the medium. Apreferred range is about 10 g/l to 100 g/l and more preferably about 40g/l to 90 g/l, e.g., 50-70 g/l.

The other preferred component of the permanganate composition is amaterial having an oxidation potential higher than the oxidationpotential of the permanganate solution and which is capable of oxidizingmanganate ions to permanganate ions. While any such oxidant may be used,it is preferred that the oxidant have an oxidation potential greaterthan about 10% higher than the oxidation potential of the permanganatesolution, and more preferably greater than about 25%, e.g., 50-100%, orhigher. Exemplary oxidants include chlorine, bromine, ozone,hypochlorite salts, m etaperiodate salts, trichloro-s-triazinetrione andits salts, and the like. Sodium hypochlorite is preferred because of itsease of use, availability, low cost and demonstrated effectiveness.Commercially available 13% by weight sodium hypochlorite solution (15%available chlorine) has provided excellent results.

The amount of oxidant employed may vary widely and, in a preferredembodiment, is correlated to the manganate ion concentration of thepermanganate solution and, most preferably, to the total manganate ionplus permanganate ion concentration. In general, for a hypochloritesalt, a range of about 1 g/l to 100 g/l or more, may be employed,preferably 5 g/l to 50 g/l, e.g., 5 g/l to 25 g/l.

It is an important feature of the invention that permanganate ions bepresent at the desired concentration in order to maintain the activityof the solution. As is well-known in the art, manganate ion is formedduring the treatment process which reduces the activity of the solutionand leads to other undesirable results such as manganese dioxide sludgeformation. The use of the secondary oxidant in the composition ineffective oxidizing amounts maintains the activity of the solution,stabilizes the solution, consumes less of the expensive permanganatesalt while consuming a secondary oxidant which, in may cases, is muchless expensive than the permanganate salt, among other benefits.

While different methods can be employed to practice the invention, it ispreferred that at desired intervals during the process the concentrationof permanganate and manganate ions be determined by analyticaltechniques. The ratio of permanganate ion concentration to the sum ofthe permanganate ion and manganate ion concentrations is calculated witha ratio greater than about 0.5 being desired, and increasing ratiosproviding, in general, a preferred process. A ratio greater than about0.7 is preferred, with ratios greater than about 0.8 and 0.9 being mosthighly preferred. Depending on the ratio, an effective amount of thesecondary oxidant is added, as needed, to the solution to control theratio at the desired level. Amounts of oxidant will vary and can bereadily determined by those skilled in the art. Preferably, the ratio isdetermined, and, if, for example, is less than 0.7, 3% by volume of a13% by weight NaOCl solution is added to the bath. After about 1 hourthe ratio is again determined and should be above the operating level of0.7. Since higher ratios are preferred, if the ratio is say between 0.8and 0.85, it has been found effective to add 1% by volume of the NaOClsolution to the bath. This will maintain the ratio, depending on usageof the bath, for an operating period of at least a day (24 hours). It ispreferred to analyze the bath on a daily basis and to make adjustmentsas needed as will be appreciated by those skilled in the art.

It is another important feature of the invention that the permanganateion concentration be also maintained at a predetermined level in thesolution, e.g., its original concentration. Permanganate ions may beadded to the solution to bring the solution to the desired permanganateion concentration based on the determined permanganate ionconcentration. It has been found that highly satisfactory operatingresults have been achieved by summing the permanganate ion and manganateion concentrations and adding permanganate ions based on this value andthis procedure is preferred based on its demonstrated effectiveness.Thus, based on potassium permanganate, if a permanganate concentrationof 65 g/l is desired and the sum of the potassium permanganate andpotassium manganate concentration is 55 g/l, 10 g/l of potassiumpermanganate would be added to the solution.

Other additives, such as wetting agents, may be used in the permanganatecomposition for special purposes as is known in the art.

The permanganate composition media is preferably aqueous andsubstantially inert with respect to the plastic being treated. Othermedia may also be employed with water being preferred for economicreasons and for its demonstrated effectiveness.

To practice the method of the invention the plastic substrate iscontacted with the permanganate composition at an elevated temperaturefor a time sufficient to render the surface receptive to the electrolessmetal plating process. Contacting procedures may vary widely andsatisfactory results for epoxy resin are provided by immersing the partin the solvent for between approximately 5 to 15 minutes at 140° to 160°F. (61° to 72° C.). The time and temperature will vary depending on thesubstrate being treated and the composition of the solution as will beappreciated by those skilled in the art. Other means such as spraying,may be used for treating the plastic part.

Other permanganate etching compositions which are suitable for use withthe solvent of step one are described in U.S. Pat. Nos, 4,042,729;4,054,693 and 4,425,380.

The treated plastic part is then rinsed to remove excess solution andmanganese residues are removed by neutralization or chemical reductionin step three using reductants such as hydrazine and oxalic acid.

The etched plastic part is now prepared for electroless metal plating byknown means as described in U.S. Pat. Nos. 2,874,072; 3,011,920;3,075,855; 3,095,309; 3,672,938 and 3,736,156; the disclosures of saidpatents being hereby incorporated by reference. In general, the plasticsurface is catalyzed employing a solution containing tin and palladiumfollowed by treatment with an accelerator solution, usually an acid, toremove excess tin and provide a palladium rich surface. The plastic isnow ready for plating with an electroless metal bath such as copper.Other methods of deposition may also be used such as vacuum vapordeposition, electrolytic plating or a combination of electroless platingand electrolytic plating.

The present invention will now be described in detail by reference tothe following examples.

EXAMPLE I

The following example illustrates the use of the improved permanganatecomposition in step two to provide an efficient, effective method forthe electroless metal plating of a printed circuit board.

A two-sided copper clad epoxy-glass FR-4 laminate board was metallizedusing the following procedure

(a) strip the copper clad from the surface of the board with 50% HN03 atroom temperature;

(b) rinse in water;

(c) clean the surface with 96% sulfuric acid for 45 seconds at roomtemperature;

(d) rinse in water for 5 minutes;

(e) immerse for 5 minutes at 140° F. (61° C.) with mild agitation in asolution comprising:

    ______________________________________                                        Sodium hydroxide         90     g                                             N-Methyl-2-pyrrolidone   100    ml                                            Ethyleneglycol monobutyl ether                                                                         20     ml                                            Water (to make)          1000   ml                                            ______________________________________                                    

(f) rinse for 5 minutes in water;

(g) etch for 10 minutes at 150° F. (66° C.) with mild agitation in asolution comprising:

    ______________________________________                                        KMnO4                 65     g                                                NaOH                  50     g                                                NaOCl                 10     g                                                Water (to make)       1000   ml                                               ______________________________________                                    

(h) rinse for 5 minutes in water;

(i) neutralize for 5 minutes at 140° F. (61.C) in a solution comprising:

    ______________________________________                                        Hydrazine.H2SO4        25     g                                               HCl (37% aqueous)      30     ml                                              Water (to make)        1000   ml                                              ______________________________________                                    

(j) rinse in water for 3 minutes;

(k) immerse in a conditioning cleaner such as ENPLATE® PC-475 for 5minutes at 145° F. (64° C.) ENPLATE PC-475 manufactured by Enthone,Incorporated, West Haven, Conn.);

(l) rinse in water for 3 minutes;

(m) immerse in a one-step palladium catalyst such as ENPLATE Activator444 for 5 minutes at 75° F. (25° C.);

(n) rinse with water;

(o) immerse in a post activator solution such as ENPLATE PA-491 for 5minutes at room temperature;

(p) rinse in water for 2 minutes;

(q) metallize in an electroless copper solution such as ENPLATE CU-700for 30 minutes at 118° F. (49° C.);

(r) rinse with water and air dry.

The metallized layer was tested for adhesion by firmly attaching a stripof adhesive tape to the metallized surface and then removing the tapewith a quick snapping motion. There was no transfer of the metallizeddeposit to the adhesive backing of the tape.

EXAMPLE II

The following example illustrates the use of the improved permanganatecomposition in step two to provide an efficient, effective method fortreating a printed circuit board.

A multi-layer FR-4 epoxy fiberglass copper clad board having drilledthrough-holes was metallized using the following procedure:

(a) immerse the board for 5 minutes at 140° F. (61° C.) in a solutioncomprising:

    ______________________________________                                        N-methyl-2-pyrrolidone   50     ml                                            Ethyleneglycol monobutyl ether                                                                         10     ml                                            Sodium hydroxide         45     g                                             Water (to make)          500    ml                                            ______________________________________                                    

(b) rinse in water;

(c) immerse the board for 10 minutes at 150° F. (66° C.) in a solutioncomprising:

    ______________________________________                                        Potassium permanganate 30     g                                               Sodium hydroxide       25     g                                               Sodium hypochlorite    5      g                                               Water (to make)        500    ml                                              ______________________________________                                    

(d) rinse in water;

(e) determine the permanganate and manganate concentrations and sodiumhydroxide concentration of the solution of step (c) at predeterminedintervals;

(f) calculate the ratio of potassium permanganate concentration to thesum of the potassium permanganate and potassium manganate concentration;

(g) add potassium permanganate and sodium hydroxide as needed tomaintain their original concentrations the amount of KMn04 being addedbased on the original KMn04 concentrations minus the sum of KM-04 andK2Mn04 concentration;

(h) add sodium hypochlorite to maintain the ratio above 0.8;

(i) repeat steps (a)-(h) until a loading of 90 ft2 panel/gallonsolution.

The board when metallized by steps (i)-(r) of EXAMPLE I producedstrongly bonded metal deposits.

The permanganate solution required 11.8g KMn04 to maintain the originalpermanganate concentration and 29g NaOCl to maintain a ratio of 0.8 orbetter. The permanganate solution had no trace of insoluble matter onthe bottom of the container.

COMPARATIVE EXAMPLE II

Example II was repeated except that the -potassium permanganate solutiondid not contain sodium hypochlorite. This solution required 15.6g KMnO₄to maintain the original permanganate concentration and the ratiodropped to 0.40. The solution had an amorphous brownish blackprecipitate at the bottom of the container.

EXAMPLE II and COMPARATIVE EXAMPLE II demonstrate the reduced level ofKMn04 needed to maintain the original concentration of the solution andthe lack of insoluble matter contaminating the solution, which mattermust be cleaned from the container. The solution of the invention hasincreased activity compared to the solution not containing NaOCl andlower operating temperature and/or shorter contact times can beemployed. Further, a separate rejuvenation procedure such as shown inthe U.S. Pat. No. 4,042,729 is not required.

EXAMPLE III

The following example illustrates the use of the improved solventcomposition in step one to enhance the adhesiveness of electrolessplated copper to epoxy resin which was cleaned with sulfuric acid.

A two-sided copper clad epoxy-glass FR-4 laminate board was metallizedusing the following procedure

(a) strip the copper clad from the surface of the board with 50% HNO3 atroom temperature;

(b) rinse in water;

(c) clean the surface with 96% sulfuric acid for 45 seconds at roomtemperature;

(d) rinse in water for 5 minutes to remove acid residues;

(e) immerse the board for 5 minutes at 145° F. (63° C.) with mildagitation in a solution comprising:

    ______________________________________                                        NaOH                     90     g                                             Dimethoxy Tetraethylene Glycol                                                                         80     g                                             Propylene Glycol Monomethyl Ether                                                                      80     g                                             Water (to make)          1000   ml                                            ______________________________________                                    

(f) rinse for 5 minutes in water;

(g) etch for 10 minutes at 150° F. (66° F.) with mild agitation in asolution comprising:

    ______________________________________                                        KMnO4                 65     g                                                NaOH                  50     g                                                Water (to make)       1000   ml                                               ______________________________________                                    

(h) rinse for 5 minutes in water;

(i) neutralize for 5 minutes at 140° F. (60° C.) in a solutioncomprising:

    ______________________________________                                        Hydrazine.H2SO4        25     g                                               HCl (37% aqueous)      30     ml                                              Water (to make)        1000   ml                                              ______________________________________                                    

(j) rinse in water for 3 minutes;

(k) immerse in conditioning cleaner ENPLATE® PC-475 for 5 minutes at145° F. (63° C.);

(l) rinse in water for 3 minutes;

(m) immerse in one-step palladium catalyst ENPLATE Activator 444 for 5minutes at 65° F. (24° C.);

(n) rinse with water;

(o) immerse in post activator solution ENPLAT PA-491 for 5 minutes atroom temperature;

(p) rinse in water for 2 minutes;

(q) metallize in electroless copper solution ENPLATE CU-700 for 30minutes at 118° F. (48° C.); and

(r) rinse with water and air dry.

The metallized layer is tested for adhesion by firmly attaching a stripof adhesive tape to the metallized surface and then removing the tapewith a quick snapping motion. There was no noticeable transfer of themetallized deposit to the adhesive backing of the tape.

COMPARATIVE EXAMPLE III

The procedure of EXAMPLE III was comparatively repeated except that step(e) (the conditioning step employing the improved solvent solution ofthe invention) was omitted. The metallized deposit was blistered and waspoorly adherent with a significant transfer of the deposit to theadhesive backing of the tape.

COMPARATIVE EXAMPLE IV

The procedure of EXAMPLE III was comparatively repeated except that thedimethoxy tetraethylene glycol component was omitted from thecomposition used in step (e). The metallized deposit was poorly adherentwith a significant transfer of the deposit being removed by the tape.

EXAMPLE IV

The procedure of EXAMPLE III was repeated on a drilled multi-layerepoxy-glass FR-4 laminate board. A standard copper micro-etch step usingENPLATE AD-485 was added between steps (l) and (m) and steps (a) and (b)were omitted. After metallization the board was electroplated in astandard acid copper plating solution to a deposit thickness of about 1mil. The board was then rinsed with water, dried and coated with anactivated rosin flux. The prepared board was then floated on moltentin-lead (60-40) solder at 550° F. (288° C.) for 10 seconds. Aftercooling the solder filled holes were cross-sectioned using standardmetallurgical techniques. Microscopic examination of the copper to epoxyinterface revealed no separation and the presence of a fine dendriticstructure.

EXAMPLE V

The procedure of EXAMPLE IV was repeated with the omission of theconcentrated sulfuric acid step (c). Microscopic examination of thecopper plating to inner layer copper foil revealed a mechanically soundconnection free of smeared resin.

EXAMPLE VI

The procedure of EXAMPLE VI was repeated substituting a 900 g/l solutionof chromic acid at 150° F. for 2 minutes for the concentrated sulfuricacid step (c). Tightly adherent plating to the epoxy hole wall surfacewas obtained.

EXAMPLE VII

The procedure of EXAMPLE VI was repeated substituting a sample of adrilled multilayer laminate that had been exposed to a plasma desmearingprocess. Tightly adherent plating to the epoxy hole wall surface wasobtained.

What is claimed is:
 1. A process for treating plastics to improve theadhesion of electroless metal plating comprising the steps of:(a)contacting the plastic with a solvent composition comprising:(i) acompound represented by the general formula ##STR3## wherein R₁ and R₂are independently selected from the group consisting of hydrogen atoms,aryl groups and alkyl groups of 1-4 carbon atoms and m is 0 to 2; and(ii) a compound represented by the general formula

    R.sub.3 --(OCH.sub.2 CH.sub.2).sub.n --O--R.sub.4

wherein R₃ and R₄ are independently selected from the group consistingof aryl groups and alkyl groups of 1-4 carbon atoms and n is 2 to 5; thecomposition comprising, by weight, about 10 g/l to saturation compound(i) and about 10 g/l to saturation compound (ii); (b) thereafter etchingthe solvent treated plastic with an alkaline permanganate solution foran effective time at an elevated temperature to improve said adhesion,said solution including a secondary oxidant capable of oxidizingmanganate ion to permanganate ion; and (c) controlling in said alkalinepermanganate solution, at desired intervals, the ratio of permanganateion concentration to the sum of the permanganate and manganate ionconcentration at a level about 0.5 by adding, as needed, an effectiveamount of the secondary oxidant to the solution.
 2. The process of claim1 wherein the ratio of permanganate ion concentration to the sum of thepermanganate and manganate ion concentration is controlled in step (c)at a value above about 0.7.
 3. The process of claim 1 wherein thepermanganate ion concentration of the alkaline permanganate solution ismaintained at a predetermined level in the solution by addingpermanganate ions to the solution.
 4. The process of claim 1 wherein thesecondary oxidant is selected from the group consisting of chlorine,bromine, ozone, hypochlorite salts, metaperiodate salts andtrichloro-s-triazinetrione salts.
 5. The process of claim 4 wherein thesecondary oxidant is NaOCl.
 6. The process of claim 1 wherein theplastic to be treated is the plastic substrate of an electronic circuitboard.
 7. The process of claim 6 wherein the plastic is epoxy.
 8. Theprocess of claim 1 wherein the alkaline permanganate solutioncomprises:(a) permanganate salt in an amount of about 10 g/l to itslimit of solubility; (b) secondary oxidant in an amount of about 1 g/lto 100 g/l; and (c) alkali metal hydroxide in an amount of about 20 g/lto 90 g/l.
 9. The process of claim 1 wherein the temperature of thealkaline permanganate solution is about 100° F. to 200° F.
 10. Theprocess of claim 1 wherein the temperature of the alkaline permanganatesolution is about 140° F. to 160° F.
 11. The process of claim 1 whereinthe alkaline permanganate solution has a pH greater than about
 13. 12.The process of claim 1 wherein the solvent composition is a swellant forthe plastic.
 13. The process of claim 1 wherein the solvent compositionhas a pH greater than about
 10. 14. The process of claim 1 wherein thesolvent composition has a pH greater than
 13. 15. A process for treatingthe epoxy substrate of printed circuit boards to improve the adhesion ofmetal plating comprising the steps of:(a) contacting the substrate withan epoxy swellant composition comprising:(i) a compound represented bythe general formula ##STR4## wherein R₁ and R₂ are independentlyselected from the group consisting of hydrogen atoms, aryl groups andalkyl groups of 1-4 carbon atoms and m is 0 to 2; and (ii) a compoundrepresented by the general formula

    R.sub.3 --(OCH.sub.2 CH.sub.2).sub.n --O--R.sub.4

wherein R₃ and R₄ are independently selected from the group consistingof aryl groups and alkly groups of 1-4 carbon atoms and n is 2 to 5; thecomposition comprising by weight, about 10 g/l to saturation compound(i) and about 10 g/l to saturation compound (ii); (b) thereafter etchingthe swellant treated epoxy substrate with a permanganate solution havinga pH greater than about 10 for an effective time at an elevatedtemperature to improve said adhesion, said solution including asecondary oxidant capable of oxidizing manganate ion to permanganateion; and (c) controlling in said permanganate solution, at desiredintervals, the ratio of permanganate ion concentration to the sum of thepermanganate and manganate ion concentration at a level above 0.7 byadding, as needed, an effective amount of the secondary oxidant to thesolution.
 16. The process of claim 15 wherein the swellant compositionhas a pH greater than about
 10. 17. The process of claim 15 wherein theswellant composition has a pH greater than
 13. 18. The process of claim16 wherein the permanganate solution has a pH greater than about
 13. 19.The process of claim 18 wherein the secondary oxidant is NaOCl.
 20. Theprocess of claim 19 wherein the permanganate ion concentration of thepermanganate solution is maintained at a predetermined level in thesolution by adding permanganate ions to the solution.
 21. The process ofclaim 20 wherein the permanganate solution comprises:(a) permanganatesalt in an amount of about 10 g/l to its limit of solubility; (b)secondary oxidant in an amount of about 1 g/l to 100 g/l; and (c) alkalimetal hydroxide in an amount of about 20 g/l to 90 g/l.
 22. The processof claim 21 wherein the temperature of the permanganate solution isabout 140° F. to 160° F.
 23. The process of claim 1 wherein said solventcomposition is an alkaline solution.
 24. The process of claim 23 whereinsaid solvent composition contains an alkali metal hydroxide in an amountof about 5 g/l to 200 g/l.
 25. The process of claim 24 wherein saidalkali metal hydroxide is sodium hydroxide.
 26. The process of claim 24wherein compound (i) is propylene glycol monomethyl ether and compound(ii) is dimethoxy tetra ethylene glycol.
 27. The process of claim 26wherein said alkali metal hydroxide, said compound (i) and said compound(ii) are each present in an amount of abut 40 to 120 g/l.
 28. Theprocess of claim 23 wherein the temperature of the solvent compositionis from about 90° F. to the lesser of the boiling point of the solutionand the softening point of the plastic, and the contact time is lessthan 30 minutes.
 29. The process of claim 28 wherein the temperature isfrom about 140° F. to 150 ° F. and the contact time is less than about10 minutes.
 30. The process of claim 15 wherein said swellantcomposition is an alkaline solution.
 31. The process of claim 30 whereinsaid solvent composition contains and alkali metal hydroxide in anamount of about 5 g/l to 200 g/l.
 32. The process of claim 31 whereinsaid alkali metal hydroxide is sodium hydroxide.
 33. The process ofclaim 31 wherein compound (i) is propylene glycol monomethyl ether andcompound (ii) is dimethoxy tetra ethylene glycol.
 34. The process ofclaim 33 wherein said alkali metal hydroxide, said compound (i) and saidcompound (ii) are each present in an amount of about 40 to 120 g/l. 35.The process of claim 30 wherein the temperature of the solventcomposition is from about 90° F. to the lesser of the boiling pint ofthe solution and the softening point of the epoxy, and the contact timeis less than 30 minutes.
 36. The process of claim 35 wherein thetemperature is from about 140° F. to 150° F. and the contact time isless than about 10 minutes.
 37. A process for treating the epoxysubstrate of printed circuit boards to improve the adhesion of metalplating comprising the steps of:(a) contacting the substrate with analkaline solution of an epoxy swellant composition comprising propyleneglycol monomethyl ether in an amount from about 10 g/l to saturation anddimethoxy tetraethylene glycol in an amount from about 10 g/l tosaturation; (b) thereafter etching the swellant treated epoxy substratewith a permanganate solution having a pH greater than about 13 for aneffective time at an elevated temperature to improve said adhesion, saidsolution including NaOCl as a secondary oxidant for oxidizing manganateion to permanganate ion; and (c) controlling in said permanganatesolution, at desired intervals, the ratio of permanganate ionconcentration to the sum of the permanganate and manganate ionconcentration at a level above 0.5 by adding, as needed, an effectiveamount of the NaOCl to the solution.
 38. The process of claim 37 whereinsaid swellant composition includes sodium hydroxide.
 39. The process ofclaim 37 wherein the swellant composition has a pH greater than about10.
 40. The process of claim 37 wherein the swellant composition has apH greater than
 13. 41. The process of claim 40 wherein the permanganateion concentration of the permanganate solution is maintained at apredetermined level in the solution by adding permanganate ions to thesolution, and the ratio of permanganate ion concentration to the sum ofthe permanganate and manganate ion concentration is controlled in step(c) at a value above about 0.7.
 42. The process of claim 41 wherein thepermanganate solution comprises:(a) permanganate salt in an amount ofabout 10 g/l to its limit of solubility; (b) NaOCl in an amount of about1 g/l to 100 g/l; and (c) alkali metal hydroxide in an amount of about20 g/l to 90 g/l.
 43. The process of claim 42 wherein the temperature ofthe permanganate solution is about 140° F. to 160° F.
 44. The process ofclaim 41 wherein the sodium hydroxide, said propylene glycol monomethylether and said dimethoxy tetra ethylene glycol are each present in saidswellant composition in an amount of about 40 to 120 g/l.
 45. Theprocess of claim 44 wherein the temperature of said swellant compositionis from about 140° F. to 150° F. and the contact time is less than 10minutes.